1. Field of the Invention
The present invention relates to a magnetooptical recording medium which records information utilizing a magnetooptic effect to ultraviolet radiation with a wavelength of less than 400 nm of a metal thin film or multilayer thin film and to a method of producing the same.
2. Description of the Related Art
Conventionally, metal films including a TbFeCo (Terbium/Iron/Cobalt) amorphous film, a Bi (bismuth)-Substitutional garnet thin film, and CoPt (Cobalt/platinum) multilayer thin film have been widely used as magnetooptical recording mediums. These thin films are significant to a polar rotation angle (hereinafter merely referred to as Kerr rotation angle) or Kerr ellipticity (e.g. more than 0.1.degree.) of a magnetooptic effect to visible rays ranging 400 to 860 nm. Those thin films have a large vertical magnetic anisotropy which provides the magnetization direction perpendicular to the thin film. Typically, the TbFeCo amorphous thin film family is practically used.
An amorphous film of TbFeCo is disclosed in a paper written by T. Suzuki, J. Appl. Phys. 69(8), pp 4756-4760 (1991) and both Bi-substitution garnet thin film and CoPt multilayer thin film are disclosed in a paper written by W. B. Zeper, F. Greidanus and P. F. Carcia; IEEE Trans. MAG25, pp 3764-3766 (1989).
There has been a strong demand to increase the recording density of the recording medium. Although a magnetooptical recording medium can record data of a higher density as compared to a conventional magnetic recording medium, further increases in the recording density of the magnetooptical recording medium are desired.
Techniques including the magnetic super-resolution method and the magnetic domain expanding method have been proposed to increase recording density. Basically, the spot diameter of a recording beam must be decreased by shortening the wavelength of light.
The TbFeCo amorphous thin film, currently and popularly used as a magnetooptical recording medium, has a Kerr rotation angle of more than 0.3.degree. adjacent to a wavelength of 760 nm. However, it is well-known that the Kerr rotation angle and Kerr ellipticity decrease as the wavelength is shortened, so that the reproduced signal strength remarkably decreases. It is predicted that shortening the wavelength of light will lead to enable optical recording in high density by ultraviolet rays. For that reason, the TbFeCo series amorphous thin film currently used cannot be used for the magnetooptical recording medium which uses a short-wavelength beam such as ultraviolet ray.
It is known that the Co-Pt multilayer thin film, or Bi-substitution garnet thin film has a large polar Kerr effect near to 400 nm or more. However, since these materials have a crystalline structure and produce large reproduction noises due to the crystalline grains, they are unsuitable in practical use.
Since the R-TM alloy thin film (where R is a rare earth element such as Tb or Dy, and TM is a transition metal such as Fe, Co, or Ni) has an amorphous structure, the grain noise is very small (by 5 to 10 dB (30 kHz band width)) and the S/N ratio is large, compared with the crystalline film. This alloy thin film is put to practical use. Much study and research has been devoted to the alloy thin film to improve the Kerr rotation angle for short wavelengths. For example, the enhancement effect of Pt is disclosed in Japanese Patent Laid-Open Publication No. Hei 5-128600 (JP-A-05-12860), while the enhancement effect of a light rare earth element is disclosed in Japanese Patent Nos. 1949740(JP-B-1949740) and 2026003(JP-B-2026003), and in Japanese Patent Laid-Open Publication Nos. 6-103621(JP-A-06-103621) and 6-60452(JP-A-06-60452).
However, with the exception of Japanese Patents No. 1949740 and 2026003, these references only discuss a wavelength region .lambda..gtoreq.400 nm (a photon energy of less than about 3 eV). Although Japanese Patents No. 1949740 and 2026003 disclose experimental data for the wavelengths reaching 200 nm, they do not disclose accurate measurements of wavelengths less than 400 nm because of light absorption. Therefore, wavelengths shorter than 400 nm have not been studied.
A magnetooptical recording medium requires a large vertical magnetization anisotropy to obtain an orientation of magnetization perpendicular to the film surface. Specifically, it is necessary to set the magnetic anisotropic constant Ku to a value larger than the demagnetizing field energy 2.pi.Ms.sup.2.
A magnetooptical recording medium which has a sufficient magnetooptic effect and a large vertical magnetic anisotropy in the ultraviolet-ray region is therefore greatly desired.